Series wiring of highly reliable light sources

ABSTRACT

The light array of this invention includes a number of columns and rows of LED&#39;s connected in a series/parallel combination. The series parallel combinations effectively optimize the impedance, accommodate failure rate, facilitate light mixing, provide a means of imbedding redundancy, and common cathodes or anodes. This arrangement provides a superior light source for consumer, industrial and specialty markets in respect to mean time between failure, process control, radiant intensity, wavelength mixing, power requirements and other characteristics of the light source. Each column includes a number of rows of plural LED&#39;s. The LED&#39;s in each row are wired in series and each column is wired in parallel so that if one LED fails only the LED&#39;s connected in series with the failed LED will also fail. There is redundancy in the circuit as well as the arrays so that if there are failures different current carrying elements or different series LEDS will automatically by powered on. The array may be connected in series with one or more LED arrays to form a module. Multiple modules may be connected in series with other multiple modules.

This invention claims the benefit of co-pending U.S. ProvisionalApplication No. 60/516,381, entitled “Series Wiring of Highly ReliableLight Sources,” filed Oct. 31, 2003, the entire disclosure of which ishereby incorporated by reference as if set forth in its entirety for allpurposes.

BACKGROUND OF THE INVENTION

Solid state lighting devices such as, for example, light emitting diodes(LED's) are used for a number of applications. One type of such solidstate lighting device is disclosed in International Patent ApplicationNo. PCT/US03/14625, filed May 28, 2003, entitled High EfficiencySolid-State Light Source And Methods Of Use And Manufacture, the detailsof which are hereby incorporated by reference.

There are numerous applications where a long string of devices, such as,for example, LED's, need to be connected electrically. Such stringspresent unique problems for the electrical engineer. On the one hand,there is a desire to string the components in series so that the currentfrom one component flows directly through the next component. This is adesired configuration because it minimizes the amount of electricalcurrent required while increasing the total voltage required across allthe components. Since high currents are more difficult to deal withbecause high currents require large gauge wires, for example, it isdesired to have lower currents and higher voltages.

However, stringing the components together in series presents a problembecause if one of the components in the string fails, it will result inthe failure of the entire string. For example, in a string of holidaylights wired in series, if one light fails the entire string also fails.To overcome this problem, holiday string lights are typically wired inparallel so that when one light fails the rest of the lights in thestring continue to operate. However, such wiring requires higher currentand lower voltage.

Wiring lights in series is preferred because the total current is lowerand the operating voltage is higher. This presents a problem because ifone light fails all lights in the series fail. Wiring lights in parallelovercomes this problem because when one light fails all other lightsstill operate. However, one undesirable aspect of wiring in parallel isthat the total current is higher and the operating voltage is lower.

One prior art approach to this problem is described in U.S. Pat. No.6,153,980 (Marshall et al). This patent describes a circuit that hasindividual sensors for each light source and can determine if any givenlight source has failed. In the event of failure, the circuit shuntscurrent around the failed component so that the rest of the componentsthat are wired in series continue to receive electrical current. Whilesuch a circuit solves the problem of allowing serial connection (and,thus, higher voltage and lower current) the circuit itself is morecomplicated, expensive, and prone to possible failure, which defeatsit's intended purpose.

What is needed is a light source that never fails or that at least hassuch a high reliability and mean time between failures that failure issomething that effectively can never happen. Thus, the preferredsolution changes from parallel wiring to series wiring forming acascading series parallel circuit substantially reducing failures andmean time between failures. The parallel/series circuitry enables theselection of current and potentials that can accommodate the specificperformance of solid state light sources in addition to complying withindustry standards for different markets. These markets can be, but arenot limited to industrial (high power), consumer (low power) andspecialty markets as in the case of aerospace and medical markets.

SUMMARY OF THE INVENTION

The present invention provides a light source that is composed of anarray of devices having a very large mean lifetime. The array is wiredin a combination series and parallel circuit that ensures that thecomposite device will virtually never burn out. The light sources in thearray of this invention are wired together in series without concern ofthe consequences of a module failure.

The array of this invention may include a composite of LED's that maynumber in the hundreds or about one thousand, for example. LED's aresolid-state light sources with very long lifetimes that are measured inhundreds of thousands of hours. The array of this invention capitalizeson the lifetime of the LED's but also capitalizes on their low operatingcurrent and voltage to produce a composite array that is partly paralleland partly in series.

The light array of this invention includes a number of columns and rowsof LED's. Each column includes a number of rows of plural LED's. TheLED's in each row are wired in series and each column is wired inparallel so that if one LED fails only the LED's connected in serieswith the failed LED will also fail. The array may be connected in serieswith one or more LED arrays.

Another advantage of the present invention is that connecting the LED'sin series provides all of the LED's in the series with the same amountof current so that the LED's have the same brightness.

This invention provides a lighting module comprising an array of LED'sconsisting of plural columns and rows, wherein each row of LED's in eachcolumn is connected in series and each column is connected in parallel.The LED array may be connected in series to one or more LED arrays. Eachcolumn in the LED array may contain at least one row of, for example,three LED's. Each column in the LED array may contain, for example,twenty-five rows of LED's. The LED array may contain, for example,thirteen columns.

This invention also provides novel circuits for driving LED's. In oneembodiment, a circuit is provided that results in a high LED peakintensity without requiring more power input. In another embodiment, acircuit is provided for pulsing an array of LED's that results in veryhigh current levels in the LED's without causing over-dissipation.

These and other embodiments are described in more detail in thefollowing detailed descriptions and the figures. The foregoing is notintended to be an exhaustive list of embodiments and features of thepresent invention. Persons skilled in the art are capable ofappreciating other embodiments and features from the following detaileddescription in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an array of LED's that are wired both in series and inparallel.

FIG. 2 shows a module of plural arrays of LED's wired together.

FIG. 3 shows a full-wave bridge rectifier for directly driving a singlestring of LED's of FIGS. 1 and 2.

FIG. 4 shows a circuit for pulsing an array of LED's as shown in FIGS. 1and 2.

DETAILED DESCRIPTION OF THE INVENTION

Representative embodiments of the present invention are shown in FIG. 1,wherein similar features share common reference numerals.

As shown in FIG. 1, an LED array 10 is shown that is wired in aseries/parallel combination. The LED array 10 includes a plurality ofindividual LED's 12 mounted on a substrate 13 and arranged in rows 14and columns 16. Each column 16 includes plural rows 14 of LED's 12 with,for example, three LED's 12 in each row 14. There may be, for example,twenty-five rows 14 in each column 16. The LED's 12 in each row 14 arewired in series and each column 16 is wired in parallel. Since the LED's12 in each row 14 are wired in series it is ensured that if one LED 12fails only the other LED's 12 in that series will fail also. The lossthe LED's 12 in a single row 14 in the total array 10 has only a minimalimpact on the total brightness of the array 10 since it consists of manyLED's 12.

In this example, the total voltage required to drive the LED array 10 isroughly three times the forward voltage drop across any given LED 12.The total current required to drive the LED array 10 is 13·25·XmA, where13 is the number of columns 16 for each array 10, 25 is the number ofrows 14 of LED's 12, and Xma is the nominal drive current required foreach LED 12. For example, the LED 12 might have a nominal forwardcurrent of 20 mA at a forward voltage of between 3.6 and 4.0 volts. Forexample, the voltage and current for driving a single board populatedwith these LED's 12 may be 13·25·0.020A=6.5A and between 10.8-12 volts.

If all of the LED's 12 were wired in parallel, the required currentwould be three times higher, and the voltage three times lower. Theconfiguration of FIG. 1 provides an improvement in offering considerablylower current at higher voltage while at the same time producing an LEDarray 10 that has a virtually unlimited lifetime.

Each LED array 10 may be wired, preferably, in series to one or moreother LED arrays to form a module as seen in FIG. 2. Multiple modulesmay be wired, preferably, in series to other multiple modules. However,because of the virtually unlimited lifetime of the LED array 10 themodules may be wired in parallel or in series without regard forconcerns that one of the LED arrays might fail causing failure of thewhole module.

For example, one might want ten LED arrays 10. Wiring them in seriesrequires (using the numbers from the above example) 6.5 amps at about120 volts. This is roughly the electrical requirement of a domesticvacuum cleaner. By comparison, if the ten LED arrays were operated inparallel they would require 65 amps at about 12 volts, which is roughlythe requirements of a light-duty arc welder. So, when wired in seriesthe electrical requirements are far more tractable than when wired inparallel.

Thus, wiring in series results in lower current and higher voltagerequirements. These requirements are more easily (cheaply andinexpensively) met by power supplies than having to provide highercurrent and lower voltage. However, as discussed above, seriesconnections result in the entire string failing when any singlecomponent fails. This is such a significant disadvantage that in almostall cases the wiring is done in parallel and the consequent cost in highcurrent and low voltage is simply absorbed by the consumer.

With the LED array of this invention, a light source is provided that ismade of distributed devices having lifetimes of hundreds of thousands ofhours. The array 10 itself is wired in a parallel/series combinationthat ensures that if one LED 12 fails, at most only two others fail withit, as shown in this example. This is a minor problem for an array withhundreds of LED's 12. Except for row 14 of LED's 12 wired in series, thecolumns 16 of LED's are wired in parallel, ensuring that the LED array 10 can virtually never fail. It is this extreme reliability that allowsmultiple LED arrays 10 to be strung together in series without regardfor failure in any given array.

The number of rows 14, columns 16, and number of LED's 12 in each row 14may vary depending on a number of factors such as, for example, the sizeof the array substrate.

FIG. 3 shows a full-wave bridge rectifier for directly driving a singlestring of LED's as shown in FIGS. 1 and 2. A resistor may be used toprovide a limit on current. One novel feature of this circuit is that nofilter capacitor is used. The LED string conducts only on the peaks ofthe pulsating-DC output of the rectifier. The LED current may be high,which may have an operational advantage in high peak light output,particularly for chemical processes. However, the duty cycle is limited.The result is a high LED peak intensity for the same power input. It isknown that the human eye responds to the peak intensity of a lightsource. The scheme of FIG. 3 results in a visible light source of higherapparent brightness for a given power dissipation.

FIG. 4 shows a novel scheme for pulsing an array of LED's as shown inFIGS. 1 and 2. In this scheme, an AC-DC supply (shown here as anoff-line rectifier) is used to charge a low-ESR (equivalent seriesresistance) capacitor to a voltage much higher than the low-currentoperating voltage of the LED. A string of LED's is placed in series witha high-current MOSFET switch across this capacitor. If the MOSFET isswitched to “ON” at a duty cycle equal to or lower than 5%, it ispossible to create very high current levels in the LED's without causingover dissipation. Since the LED output is proportional to current in theLED, the resulting peak optical output of the LED is many times its DCvalue. This can have advantages both in visible and chemical systemsapplications.

An LED can be electrically modeled as a diode with a series resistance.Pulsing the LED in the manner described overcomes the series resistanceand allows the current in the LED to be determined by the usual diodeequation:I=Is exp (V/kt),where I is the current in the LED, Is is the saturation current, V isthe voltage applied across the diode junction (not the LED), k is theBoltzman constant, and t is the absolute temperature.

It can be shown that very high currents are possible in an LED junctionif the series resistance can be overcome by high-voltage pulsing means.Voltages across individual LED's can be in excess of 20 volts for a3-volt junction voltage. The actual construction of the individual LEDwill determine how high the applied voltage can be before voltagebreakdown occurs. As such, voltages considerably higher than a typical3.3 volts may be applied to drive the LED's. Individual LED's may bepulsed with voltages of between 6-50 volts. However, voltages up to 150volts may be applied to the LED's. It is also possible with thisinvention to pulse at least one LED up to 1,000 times its DC currentvalue.

Persons skilled in the art will recognize that many modifications andvariations are possible in the details, materials, and arrangements ofthe parts and actions which have been described and illustrated in orderto explain the nature of this invention and that such modifications andvariations do not depart from the spirit and scope of the teachings andclaims contained therein.

While the inventor understands that claims are not a necessary componentof a provisional patent application, and therefore has not includeddetailed claims, the inventor reserves the right to claim, withoutlimitation, at least the following subject matter.

1. A lighting device, comprising, an array of LED's consisting of pluralcolumns and rows, wherein each row of LED's in each column is connectedin series and each column is connected in parallel.
 2. The lightingdevice of claim 1, wherein the LED array is connected in series to oneor more LED arrays to form a module.
 3. The lighting device of claim 1,wherein each column in the LED array contains at least one row of one ormore LED's.
 4. The lighting device of claim 3, wherein each column inthe LED array contains at least two or more rows of LED's.
 5. Thelighting device of claim 4, wherein the LED array contains at least twoor more columns.
 6. The lighting device of claim 1, wherein the LED'sconnected in series are supplied with the same amount of current so thateach LED emits the same brightness.
 7. The lighting device of claim 1,wherein each of the two or more LED's in each column is also suppliedwith the same amount of current so that each column emits the samebrightness.
 8. The lighting device 3, wherein each module is connectedin series to one or more modules.
 9. The lighting device 3, wherein eachmodule is connected in parallel to one or more modules.
 10. A method ofmaking a lighting device, comprising, providing an array of LED'sconsisting of plural columns and rows, wiring each row of LED's in eachcolumn in series, and wiring each column in parallel.
 11. The method ofclaim 10, comprising, connecting the LED array in series or parallel toone or more LED arrays to form a module.
 12. The method of claim 10,comprising, providing the LED's connected in series with the same amountof current so that each LED emits the same brightness.
 13. The method ofclaim 10, comprising, providing the LED's connected in parallel with thesame amount of current so that each LED emits the same brightness. 14.The lighting device of claim 1, wherein the LED's are driven by afull-wave bridge rectifier circuit.
 15. The lighting device of claim 1,wherein the LED's are driven by a circuit in which an AC-DC supply isused to charge a low-ESR capacitor to a voltage that is substantiallyhigher than the low-current operating voltage of the LED.
 16. Thelighting device of claim 15, wherein a string of LED's is placed inseries with a high-current MOSFET switch across the capacitor.
 17. Themethod of claim 10, wherein the LED's are driven by a full-wave bridgerectifier circuit.
 18. The method of claim 10, wherein the LED's aredriven by a circuit in which an AC-DC supply is used to charge a low-ESRcapacitor to a voltage that is substantially higher than the low-currentoperating voltage of the LED.
 19. The method of claim 18, wherein astring of LED's is placed in series with a high-current MOSFET switchacross the capacitor.